1. Field of the Invention
The present invention relates to beam guiding systems, imaging methods, an electron microscopy system and an electron lithography system.
2. Brief Description of Related Art
Conventional electron microscopy systems include electron microscopes of the SEM type (scanning electron microscope), of the SEEM type (secondary electron emission microscope) or of the LEEM type (low energy electron microscope).
In electron microscopes of the SEM type, a finely focused probe-forming primary electron beam is scanned across the object surface, and an integral secondary electron intensity is detected dependent upon the position of the finely focused primary electron beam on the object plane in order to obtain an electron-optical image of the object.
In electron microscopes of the SEEM-type and LEEM type, respectively, an extended region or field is illuminated on the object surface by a primary electron beam, and secondary electrons emanating from this field are imaged by means of an electron optics on a position-sensitive detector in an image-preserving manner in order to obtain an electron-optical image of the object on the detector.
An advantage of the SEM-type microscope is that a resolution of the image is only restricted by a focusing quality of the primary electron beam. A disadvantage of the SEM type microscope is a low throughput at a high resolution imaging since the object surface has to be scanned point-by-point or pixel-by-pixel, respectively.
An advantage of the LEEM-type and SEEM-type microscopes, respectively, lies in the possibility to simultaneously obtain image information of an extended region or field. A disadvantage of the same is a limitation of the resolution by a quality of the imaging optics. The quality of such imaging optics decreases with increasing extension of the imaged field.
In an electron microscopy system according to co-pending US 2003/0066961 A1, a displaceable extended field of the object surface is imaged on a position-sensitive detector.
FIG. 1 schematically shows such electron microscopy system 1. It is used to image an object 5 positioned in an object plane 3 of the microscopy system 1 onto a position-sensitive detector 7. To this end, the microscopy system 1 comprises a microscopy optics 11 which provides a beam path for secondary electrons in order to electron-optically image a region 13 of the object plane 3 onto the detector 7. The region 13 which is imaged onto the detector 7 is displaceable with respect to an optical axis 17 of the microscopy optics 11, a possible displacement of the region 13 from the optical axis 17 being designated by M in FIG. 1.
The secondary electrons emanating from the region 13 are formed to a secondary electron beam 14 which is supplied to the detector 7 by the microscopy optics 11. To this end, the microscopy optics comprises plural components centered on optical axis 17, namely an objective lens 19, a field lens 21 and a further magnification optics 23. Between the objective lens 19 and the field lens 21, there are disposed two beam deflectors 25 and 27 spaced apart from each other along the optical axis 17. They are controlled by a controller 29 to each provide an adjustable deflection angle of a value β for the secondary electron beam 14. However, the deflection angles provided by the two beam deflectors 25 and 27 are opposite in sign so that the secondary electron beam 14 passes straightly through the two beam deflectors, but with an adjustable displacement M.
The secondary electrons are released from the object 5 by a primary electron beam 33 which is generated by an electron source 35, collimated by a collimating lens 37, shaped by an aperture stop 39 and supplied to a beam splitter/combiner 41. The beam splitter/combiner 41 superimposes a beam path of the primary electron beam 33 with a beam path of the secondary electron beam 14. The primary electron beam 33 passes through the field lens 21, the beam deflectors 25, 27 and the objective lens 19. The primary electron beam 33 is also deflected by the deflectors 25, 27, but not necessarily at exactly the same angles β as the secondary electron beam 14. Rather, it is sufficient for the primary electron beam 33 to illuminate the field 13 which is imaged on the detector 7 merely fairly homogeneously. Accordingly, the demands on the imaging properties of the optical system 11 for the primary electron beam 33 are less restrictive than the demands on the imaging properties of the optical system 11 for the secondary electron beam 14.
The objective lens 19 provides a focusing field for the secondary electron beam 14, the optical axis 31 of the focusing field being displaceable relative to the otpical axis 17. The controller 29 controls the objective lens 19 such that the optical axis 31 thereof coincides with the beam displacement M provided by the beam deflectors 25, 27 and such that the optical axis 31 centrally intersects the region imaged on the detector 7, respectively.
An example of a magnetic round lens having a laterally displaceable axis is disclosed in the article by Goto et al., “MOL (Moving Objective Lens)”, Optic 48 (1977), pages 255 et seq., or in U.S. Pat. No. 4,376,249.
In such MOL lens a maximum displacement M of a large extended field 13 is small due to a limited structural volume of the lens and a limited working distance between the object plane and the MOL lens. Therefore, a combination of a “comb lens” and a slit lens was proposed in order to imitate a round lens field having an optical axis which is displaceable over greater distances.
Such an objective lens 19 formed to imitate a displaceable round lens field by a combination of comb lens and slit lens is schematically shown in a perspective partial view of FIG. 2.
The combination comprises three lenses arranged at a distance from each other in z-direction, namely a slit electrode 43 having an aperture 45 elongated in x-direction, at the bottom; a slit electrode 47 disposed uppermost in z-direction which likewise comprises an aperture 49 elongated in x-direction; and a comb lens 51 disposed between the two slit electrodes 43 and 47. The comb lens 51 comprises two rows of finger electrodes 53 which are disposed on both sides of a central axis 55 of the comb lens 51 and extending in x-direction.
Electric potentials are supplied to the two slit electrodes 43 and 47 as well as to the finger electrodes 53 by a controller, not shown in FIG. 1, so that adjustable electric fields can be produced between the electrodes 43, 47 and 53. They act on a beam of charged particles which is oriented transversely to the xy-plane and passes through the apertures of the lenses 47, 51 and 45. When an electric potential is applied to the slit electrodes 43 or 47 which is different from the potential of the beam of charged particles in the plane of the slit electrodes 43, 47, the slit electrodes 43 and 47, respectively, have a same effect on the beam as a cylinder lens. An orientation of the electric field lines of a cylinder lens field as it is generated by such a slit electrode 43, 47 is schematically shown in FIG. 3a. 
A pattern of electric potentials or voltages may be supplied to the finger electrodes 53 of the comb lens 51 such that an electric quadrupole field is generated in the aperture of the comb lens 51. A configuration of field lines of such quadrupole field is schematically shown in FIG. 3b, the field having an axis of symmetry 57 which extends in z-direction and intersects the longitudinal axis 55 of the comb lens 51.
A beam of electrically negatively charged particles entering this quadrupole field is focused in x-direction and defocused in y-direction.
Accordingly, when a beam enters the objective 19 along the axis of symmetry 57 of the quadrupole field, it is as a whole subjected to the effects of the cylinder lens fields provided by the slit electrodes 43 and 47 according to FIG. 3a and of the quadrupole field provided by the comb lens 51 according to FIG. 3b. The beam is thus subjected to a superposition of the field configurations shown in FIGS. 3a and 3b and, if the strengths of the cylinder lens fields and the quadrupole field are appropriately adjusted to each other, the same effect is provided on the beam as that provided by a round lens field symmetrically disposed in respect of the axis of symmetry 57, the field lines of such a field being schematically illustrated in FIG. 3c. 
If appropriate voltages are applied to the electrodes 43, 47 and 53, it is thus possible to focus a beam of charged particles by means of the objective 19 in a similar way as it is effected with a round lens, wherein the optical axis 57 of the round lens is displaceable in x-direction.
It has been found that an imaging quality achievable with such an objective lens does not satisfy higher demands in particular in situations where object fields of a larger extension are simultaneously imaged.